Marina Del Rey is an aberrant Los Angeles neighbourhood. It is mostly uncrowded, the air is sharp and clean, the sky blue and actually visible at least 250 days of the year, the streets safe. Democratic in an upmarket sort of way, the harbour here is host to 60-foot yachts and celebrity sailboats as well as a handful of floating wooden barges that are unseaworthy eyesores to some, but comfy homes to a collection of artists and harmless eccentrics.

It's hardly a place to launch a mass murder.

But that is apparently what Kathryn Schoonover had in mind last Sunday afternoon. The frail 50-year-old cancer patient, her head bald as the result of chemotherapy treatments, stood quietly at a public counter in a post office, preparing a stack of grey envelopes for the mail.

The giveaway was her protective gloves. Or perhaps it was the fact that Ms Schoonover was methodically filling the envelopes with little plastic bags of white powder, spooned from a canister marked POISON.

Whichever, it was enough to cause another customer to hurry away and phone the police. It turned out that Ms Schoonover was attempting to post packets of cyanide, a deadly poison that can cause asphyxiation within minutes, that were disguised as nutritional supplements.

In Ms Schoonover's van police found a paper marked "hit list" of 100 names and addresses. Most were doctors, nurses, social workers and police officers.

On Tuesday, some 3,000 miles away in Niskayuna, New York, Kathleen Elbrecht picked up the telephone to hear a California police officer warn her that her name was on the hit list and to watch for strange mail. Ms Elbrecht, who works as a nurse manager at a psychiatric centre in Albany, New York, immediately thought about the odd package she had received in May.

"I think I already got one," she told the officer. It had arrived advertised as a vitamin supplement. Ms Elbrecht dipped her finger in the powder and put a crystal on her tongue. "It was very nasty, bitter," she told The Albany Times Union newspaper. Ms Elbrecht got a headache and a tightening in the throat that disappeared in two hours, but dismissed the incident as some kind of mistaken mailing.

Instead, it emerged this week that Ms Schoonover was on a campaign of revenge against those she felt had wronged her, people who when contacted by police could barely remember they had ever met her.

Her list was large but specific. Ms Elbrecht, for example, worked at the mental facility where Ms Schoonover had been treated in June 1994. Several police officers on the list had been involved in stopping Ms Schoonover for traffic violations, which resulted in the loss of her driving licence.

Although little is known about her, it is known that once upon a time Kathryn Schoonover was a pretty normal person, a bank manager at a branch of First Interstate, a large California bank. Somehow, somewhere along the line, she came apart.

In doing so, Ms Schoonover has much company.

Frustrated, wronged, rejected or unappreciated, a growing number of people are expressing their rage by sending bombs or poisons. A culture that has in the last decade seemed to celebrate victimisation now seems to have created a deadly response call, the notion that being treated unfairly is just cause for retribution.

(Perhaps its genesis goes back to 1979, when US President Jimmy Carter shortened his tenure by telling the American people that "Life is unfair").

That doctor with a rude bedside manner, the boss who didn't approve that pay raise, the insurance company that put the customer on hold for 20 minutes on the telephone . . . all have been targets in recent months. It is a trend that has provided an eerie growth in the number of active bomb squads that are a part of many American local police departments.

The guy skilled at diffusing explosives used to be a fairly idle character in most cities' police buildings. Now, whole departments are springing up, equipped with sophisticated robots and featuring experts trained by the US Army.

The New Hartford, Connecticut Police Department Bomb Squad has its own website, as does the Boston Police Department EOU (Explosive Ordnance Unit). In Boston the 17-member bomb squad responded to 250 calls in 1997. They dealt with 132 bomb threats, 103 devices or hoax devices and 15 military-strength bombs.

In Los Angeles, some 86 bombs were found in 1995. That number jumped to 175 bombs, actual explosives, found in just the first half of 1996.

The idea that only deranged and isolated psychopaths hunkered down in wilderness cabins, such as "Unabomber" Theodore Kaczynski keep the bomb squads busy might be a slight comfort. But it would be mistaken. Two recent examples are sadly more representative.

In March Sheila Rockwell (52) opened the door of her small ranch house in Fair Haven, Vermont, to receive a package for her 17-year-old son Christopher. Later, seated beside his computer, Christopher opened the cardboard box, thinking it was some computer parts.

A pipe bomb exploded, ripping through Christopher's abdomen and left leg. Mrs Rockwell, a few feet away, had the tips of her fingers blown away and a three-inch piece of metal pipe embedded in her knee. Christopher died a few hours later.

Police soon arrested a 35-year-old truck driver named Chris Dean in Indiana. Dean had bought some radio parts from the teenager over the Internet and was angry and dissatisfied with his purchase. Never in trouble with the law before, described by a former boss as "an excellent person", Dean had downloaded a simple bomb recipe from the Internet - pipe, powder, a clothespin and some thumb tacks. Dean told a friend he was going to mail the boy who cheated him a "surprise" package.

In May a 15-year-old boy described as a good student, Kipland Kinkel, shot his mother and father in Springfield, Oregon before heading over to Thurston High School. There he fired 51 rounds from a semi-automatic gun into a cafeteria, killing two teenagers and wounding 22 others.

When police when to the boy's home, they found four "sophisticated" home-made bombs. One was so large the bomb squad had to remove it from the home before defusing it. Also found was a hand grenade. The boy, a popular student known for his jokes, had listed his occupation on his Internet service account as "student, surfing the Web for info on how to build bombs".

Kinkel's dispute with his parents and the students at his school may never be known, much less understood. But his rage, and his easy access to materials and how-to manuals, are not so different from the 50-year old cancer patient in Los Angeles. Moreover the trend in revenge is even seen in Japan, where a spate of shootings and poisoning has frayed nerves.

Earlier this month four people died in Wakayama when cyanide was mixed in rice bowls at a festival. On Thursday, bottles of a lethal disinfectant disguised as a diet drink were sent to 23 students and a teacher at a Tokyo high school. One boy was hospitalised with a seriously burned throat.

For now the vengeful person - wronged or mad - is still, like Ms Schoonover was until a few days ago, invisible.